Devices such as power shovels, loaders, bulldozers, hydraulic lifts, and the like rely on hydraulic cylinders and motors in order to perform their various functions. The hydraulic cylinders or motors typically are powered by a hydraulic pump, such as a variable displacement pump, which is connected through a directional control valve generally operated directly or indirectly by manually manipulated handles or the like which control flow of hydraulic fluid to the hydraulic cylinders or motors.
Directional control valves heretofore have generally included a valve body having a pressure port which is connected to the pump, tank ports which are connected to a tank or reservoir for hydraulic fluid, and work ports connected to one or more hydraulic cylinders. Operation of the control valve selectively connects various ports with one another in order to control operation of the hydraulic cylinders so that fluid is delivered to the cylinders and exhausted from the cylinders.
A typical fluid control valve has a bore formed in the valve body and a valve spool that can be controllably shifted in the bore by suitable means, such as through fluid actuation, or use of a solenoid(s), mechanical linkage(s), etc. The spool has a plurality of circumferential grooves and the valve body has various ports in communication with the bore via passageways that are selectively connected by positioning the spool axially within the bore.
The directional control valves may be employed in load sensing systems wherein the pump that generates the flow of fluid to the fluid control valve (or valves) delivers that fluid at a variable flow rate and at a variable output pressure based upon the instantaneous requirements of the device controlled by hydraulic cylinder(s)/motor(s) connected to the directional control valve. That is, a load sense signal may be used, for example, to control a variable displacement pump so that displacement volume of the pump can be varied to accommodate varying load conditions. The load sense signal acts as a feedback signal to the pump which is representative of the pressure of the fluid being supplied to the consuming device. Directional control valves that provide such a feedback signal are generally referred to as load sensing valves.
In some load sensing systems, the load sense signal will be at zero or a nominal pressure when the control valve is in a null position. Actuation of the control valve out of its null position will cause pressurized fluid from the pump to be supplied to one of the working ports while allowing for return flow through the other working port. When this occurs, the load sense signal will rapidly increase so as to be indicative of fluid pressure being supplied to the working port and thus the load on the system. In some systems the load sense signal that tracks the pressure supplied to the hydraulic cylinder/motor may be higher than the actual pressure supplied, i.e. maintained at a system margin pressure.
For smoother operation, provision has been made for boosting the load sense signal upon the valve shifting to supply fluid pressure to one of the working ports.